In an automatic transmission using planetary gear mechanisms, a plurality of frictional engaging elements for binding plural sets of planetary gear units and a plurality of rotational elements constituting these planetary gear units are incorporated in a transmission input shaft connected to a crank shaft of an engine through a torque converter, and combinations of engaging conditions of these frictional engaging elements are changed over by way of a hydraulic pressure to achieve a desired shift position.
For this case, an automatic transmission is proposed in Japanese Patent Publication Laid-open (Japanese OPI) 63-210443, 63-210445, and others in which a plurality of electromagnetic change-over valves individually duty controlled are incorporated in the individual frictional engaging elements and electronically controlled to independently control hydraulic pressures supplied to the plurality of frictional engaging elements.
In such an automatic transmission, an increased number of electromagnetic change-over valves, which are considerably more expensive than a spool valve, must be used in association with an increase in the number of shift positions, leading to an increase in parts cost.
Then, a type is proposed in Japanese OPI 62-41466 and others, in which a hydraulic pressure change-over valve is disposed between an electromagnetic valve and two frictional engaging elements, and hydraulic oil is supplied by the hydraulic pressure change-over valve to one of the two frictional engaging elements, thereby reducing the number of electromagnetic change-over valves.
The hydraulic control apparatus for an automatic transmission disclosed in Japanese OPI 62-41466 and others is provided with a shuttle valve for changing over a single electromagnetic valve to forward frictional engaging elements and a direct-connecting clutch, and the shuttle valve is to change over a signal pressure to a control valve for controlling hydraulic pressure supplied to the individual frictional engaging elements.
In consideration of abrasion resistance of facing and the like of frictional engaging elements of an automatic transmission and a decrease in shift-time, it is preferable to vary hydraulic pressure supplied to these frictional engaging elements according to the magnitude of driving torque of the engine. That is, it is preferable to set the hydraulic pressure supplied to the frictional engaging elements to a large value in a low-speed stage where the driving torque of the engine is relatively large to prevent slipping in a frictional engaging element in the engaged condition, and to decrease the hydraulic pressure of hydraulic oil (hereinafter referred to as "line pressure") supplied from an oil pump into a hydraulic pressure circuit to decrease a loss in the oil pump in a high-speed stage where the driving torque of the engine is relatively small.
For this purpose, a hydraulic control apparatus for an automatic transmission proposed in Japanese OPI 63-210445, for example, is provided with a hydraulic pressure change-over valve which, when a high-speed side position is selected, supplies a pilot pressure to a regulator valve for reducing hydraulic oil from the oil pump to a predetermined line pressure to reduce the line pressure and, when a high-speed side position is selected, supplies a pilot pressure from the hydraulic pressure change-over valve to the regulator valve to reduce the line pressure supplied from the regulator valve to individual frictional engaging elements.
However, in the automatic transmission disclosed in Japanese OPI 63-210445, an increased number of electromagnetic change-over valves, which are considerably more expensive than a spool valve, must be used in association with an increase in the number of shift positions, coupled with requirement for the hydraulic pressure change-over valve, leading to an increase in cost.
In this regard, the apparatus disclosed in Japanese OPI 62-41466 is possible to reduce the number of electromagnetic change-over valves, but does not disclose a technology to change over the line pressure between the low-speed and high-speed side shift positions.